300-410 IPv6 Traffic Filtering and uRPF Practice Question
This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A network engineer runs the following command on Router R1:
R1# show ipv6 access-list FILTER-IPv6
IPv6 access list FILTER-IPv6
permit ipv6 2001:DB8:1::/48 any sequence 10
deny ipv6 any any sequence 20
Based on this output, what is the effect of this access list when applied to an interface?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
It permits only IPv6 traffic from 2001:DB8:1::/48 and denies everything else
The access list FILTER-IPv6 has two entries: a permit statement for source 2001:DB8:1::/48 to any destination (sequence 10), followed by an implicit deny all (sequence 20). When applied to an interface, only traffic matching the permit entry is allowed; all other IPv6 traffic is denied by the implicit deny rule at the end of the list. This results in permitting only traffic from the specified prefix and denying everything else.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
It permits all IPv6 traffic
Why it's wrong here
Only traffic from the specified prefix is permitted; other traffic is denied.
✗
It denies all IPv6 traffic from 2001:DB8:1::/48
Why it's wrong here
The first line permits that prefix.
✓
It permits only IPv6 traffic from 2001:DB8:1::/48 and denies everything else
Why this is correct
Sequence 10 permits the prefix, sequence 20 denies all other traffic.
Related concept
Read the scenario before looking for a memorised answer.
✗
It permits all IPv6 traffic except from 2001:DB8:1::/48
Why it's wrong here
The ACL does the opposite – it permits that prefix and denies all else.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the implicit deny all at the end of an access list, and the trap here is that candidates may overlook the deny ipv6 any any entry (sequence 20) or assume it is not present, leading them to incorrectly think the ACL permits all traffic (Option A) or permits all except the specified prefix (Option D).
Detailed technical explanation
How to think about this question
IPv6 access lists operate similarly to IPv4 ACLs, with an implicit deny all at the end unless overridden by a permit any any statement. The sequence numbers (10 and 20) determine the order of evaluation; Cisco IOS processes entries in ascending sequence order, stopping at the first match. In real-world scenarios, this ACL might be applied inbound on an interface to restrict traffic from a specific customer prefix while blocking all other IPv6 traffic, but note that it does not filter based on destination, only source.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
IPv6 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: It permits only IPv6 traffic from 2001:DB8:1::/48 and denies everything else — The access list FILTER-IPv6 has two entries: a permit statement for source 2001:DB8:1::/48 to any destination (sequence 10), followed by an implicit deny all (sequence 20). When applied to an interface, only traffic matching the permit entry is allowed; all other IPv6 traffic is denied by the implicit deny rule at the end of the list. This results in permitting only traffic from the specified prefix and denying everything else.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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